DMA support

[extrapilot]

Some libraries could benefit from DMA support as an option. I know that the ECAN library leverages DMA already, but I think the SD and Uart libraries could see major performance improvements. For example, with an SD block write, at high speed you might be hooked in for 3-4ms per 512bytes. But if the MMS_Init is already called, then sending a DMA buffer via SPI would be trivial to implement.

Not sure if the SPI library would be as easy to implement, since you may need to parse replys on a byte basis. But it would also be a nice option- in a case where some block data transfers (EEprom comes to mind) are needed.

Best
Rob